Caffeine and Exercise PerformanceCaffeine may be the most widely used stimulant in the world. It is found in a variety ofplants, dietary sources (including coffee, tea, chocolate, cocoa, and colas), and nonprescriptionmedications. The average caffeine consumption in the USA is approximately 2cups of coffee per day (200 mg); 10% of the population ingests more than 1000 mg per day.Caffeine is a socially acceptable, legal drug consumed by all groups in society.Caffeine is often referred to as a nutritional ergogenic aid, but it has no nutritional value.Ingested caffeine is quickly absorbed from the stomach and peaks in the blood in 1-2 hours.Caffeine has the potential to affect all systems of the body, as it is absorbed by most tissue.The remaining caffeine is broken down in the liver and byproducts are excreted in urine.CAFFEINE AND ENDURANCE EXERCISE PERFORMANCELaboratory studies from the 1970’s suggested that caffeine enhanced enduranceperformance by increasing the release of adrenaline into the blood stimulating the release offree fatty acids from fat tissue and/or skeletal muscle. The working muscles use this extra fatearly in exercise, reducing the need to use muscle carbohydrate (glycogen). The “sparing”of muscle glycogen made more available later in exercise to delay fatigue.In the 1980’s, many studies found that caffeine did not alter exercise metabolism, andimplied that it had no ergogenic effect, without actually measuring performance. A fewreports did examine caffeine and performance during endurance exercise and generallyfound no ergogenic effect. By the end of the decade, it was suggested that caffeine did notalter metabolism during endurance exercise and may not be ergogenic.Recent work reported that ingestion of 3-9 mg of caffeine per kilogram (kg) of body weightone hour prior to exercise increased endurance running and cycling performance in thelaboratory. To put this into perspective, 3 mg per kg body weight equals approximately onemug or 2 regular size cups of drip-percolated coffee; and 9 mg/kg = approximately 3 mugsof 5-6 regular size cups of coffee. These studies employed well-trained, elite or serious,recreational athletes. Studies with untrained individuals cannot be performed due to theirinability to reliably exercise to exhaustion.The mechanism to explain these endurance improvements is unclear. Muscle glycogen isspared early during submaximal exercise following caffeine ingestion (5-9 mg/kg). It isunknown whether glycogen sparing occurs as a result of caffeine’s ability to increase fatavailability for skeletal muscle use. Furthermore, there is no evidence supporting ametabolic component for enhancing performance at a low caffeine dose (3 mg/kg).Therefore, it appears that alterations in muscle metabolism alone cannot fully explain theergogenic effect of caffeine during endurance exercise.CAFFEINE AND SHORT-TERM EXERCISE PERFORMANCEResearch suggests that caffeine ingestion improves performance during short-term exerciselasting approximately 5 minutes at 90 to 100 percent of maximal oxygen uptake in thelaboratory. This exercise intensity requires maximal provision of energy from both aerobic(oxygen requiring) and anaerobic (non-oxygen) sources. It is unknown if this finding appliesto race situations. The reasons for the performance improvement may be a direct positiveeffect of caffeine on muscle anaerobic energy provision and contraction or a central nervouscomponent related to the sensation of effort. Caffeine ingestion does not appear to improvesprint performance, but additional well-controlled laboratory and field studies are required toconfirm this conclusion. Sprinting is defined as exercise that can be maintained from a fewseconds to 90 seconds where most of the required energy is derived from anaerobicmetabolism.ACSM CURRENT COMMENT PRACTICAL ASPECTS OF CAFFEINE INJECTIONCaffeine Dose. Caffeine is a “controlled or restricted substance” as defined by theInternational Olympic Committee (IOC). Athletes are allowed up to 12 ug caffeine permilliliter of urine before it is considered illegal. The acceptable limit in sports sanctioned bythe National Collegiate Athletic Association (NCAA) in the U.S. is 15 ug/ml urine. These highurinary limits are to allow athletes to consume normal amounts of caffeine prior tocompetition. A large amount of caffeine can be ingested before reaching the “illegal” limit.For example, if a 70 kg person rapidly drank about 3-4 mugs, or 5-6 regular size cups ofdrip-percolated coffee (~9 mg/kg bw) one hour before exercise, exercised for 1-1.5 hoursand then gave a urine sample, the urinary caffeine level would only approach the limit (12ug/ml). The odds of reaching the limit through normal caffeine ingestion are low, exceptwhere smaller volumes of coffee with very high caffeine concentrations are consumed.Therefore, an illegal urinary caffeine level makes it highly probable that the athletedeliberately took supplementary caffeine tablets or suppositories in an attempt to improveperformance.The optimal dose for maximizing the chance that exercise performance will be enhanced is~3 – 6 mg/kg, where side effects are minimized and urine levels are legal. The side effectsof caffeine ingestion include anxiety, jitters, inability to focus, gastrointestinal unrest,insomnia, irritability, and, with higher doses, the risk of heart arrhythmias and mildhallucinations. While the side effects associated with doses of up to 9 mg/kg do not appearto be dangerous, they can be disconcerting if present prior to a competition and may impairperformance. Ingestion of higher doses of caffeine (10-15 mg/kg) is not recommended asthe side effects worsen. It should also be noted that most studies have used pure caffeinerather than a caffeinated beverage or food. Thus, it is not certain that consuming the“equivalent dose of caffeine” as coffee, for example, will have the same result.Diuretic Effect of Caffeine. Coffee and/or caffeine are often reported to be diuretics,suggesting that ingestion of large quantities could lead to poor hydration status prior to andduring exercise.However, the available literature does not support immediate diuretic effect as body coretemperature, sweat loss, plasma volume and urine volume were unchanged during exercisefollowing caffeine ingestion.Ethical Considerations. It is easy for endurance athletes to improve performance “legally”with caffeine, as ergogenic effects have been reported with as little as 3 mg/kg body weight(bw). Even ingesting a moderate caffeine dose (5-6 mg/kg) is permissible. It has beensuggested that caffeine should be banned prior to endurance competitions, requiring theathletes to abstain from caffeine approx. 48-72 hours before competition. This limitationwould ensure that no athlete had an unfair advantage on race day, but would not preventcaffeine use in training. However, even if caffeine is banned in the future, what practiceshould athletes follow at present? For elite athletes, it is currently acceptable andreasonable to have their normal dietary coffee. However, if they deliberately take purecaffeine to gain an advantage on competitors, it is clearly unethical and is considereddoping.An equally important issue is the use of caffeine by the average active teenager or adult.Caffeine’s widespread use was demonstrated in a recent survey by the Canadian Centre forDrug Free Sport. The survey found that 27% of Canadian youths (11-18 years old) had useda caffeine-containing substance in the previous year for the specific purpose of enhancingathletic performance. Does caffeine act as a “gateway” drug for the young who then usedangerous substances? For the average, active teenager or adult who is exercising with thegoals of enjoyment and self-improvement, using caffeine defeats these purposes. Propertraining and nutritional habits are more sensible and productive approaches.ACSM CURRENT COMMENT SUMMARYCaffeine ingestion (3-9 mg/kg bw) prior to exercise increases performance during prolongedendurance exercise and short-term intense exercise lasting approx. 5 minutes in thelaboratory. These results are generally reported in well-trained elite or recreational athletes,but field studies are required to test caffeine’s ergogenic potency in the athletic world.Caffeine does not appear to enhance performance during sprinting lasting less than 90seconds, although research in this area is lacking. The mechanisms for improved endurancehave not been clearly established. Muscle glycogen sparing occurs early during enduranceexercise following caffeine ingestion but it is unclear whether this is due to increased fatmobilization and use by the muscle. The positive effect of caffeine during exercise lastingapprox. 5 minutes is not related to the sparing of muscle glycogen. The ergogenic effects ofcaffeine are present with urinary caffeine levels that are well below the IOC allowable limit(12 ug/ml). This raises ethical issues regarding caffeine use in athletics. Should the practicebe condoned, as it is legal, or should it be discouraged, as it promotes the “doping mentality”and may lead to more serious abuse? One solution would be to add caffeine to the list ofbanned substances, thereby requiring athletes to abstain from caffeine ingestion 48-72hours prior to competition and discouraging its use as a doping agent to increaseperformance in the average population.
Written for the American College of Sports Medicine byLawrence L. Spriet, Ph.D., FACSM (Chair) and Terry E. Graham, Ph.D., FACSM.
Written for the American College of Sports Medicine byLawrence L. Spriet, Ph.D., FACSM (Chair) and Terry E. Graham, Ph.D., FACSM.
